1. Field of the Invention
This invention relates to surface-active difunctional and multifunctional (i.e., with more than two carbodiimide groups) carbodiimides and their preparation and use as dispersing agents for polycarbodiimides which are useful as cross-linking agents for carboxyl-containing organic resins, preferably latex resins, or neutralized, carboxylated water-soluble resins, and carboxylated solution resins.
2. Summary of the Prior Art
Carbodiimides are a well-known class of organic compounds. Dicyclohexylcarbodiimide has been useful for many years as a condensation agent in the preparation of peptides, as described by Sheelan and Hess (J. Chem. Soc., 77, 1067 (1955)). Monodisperse, difunctional carbodiimide compounds have been prepared by the desulfurization of thioureas using hypochlorite. Such compounds were prepared, for example, by Iwakura, et al. (Makromol. Chem., 98, 21 (1966); Bull. Chem. Soc. Jpn., 40, 2383 (1967)). Multifunctional, linear, polydisperse polycarbodiimides have been prepared by Campbell from diisocyanatoalkanes or diisocyanatoarenes using a phospholine oxide catalyst (U.S. Pat. No. 2,941,966 (1960)). Use of sodium hypochlorite to desulfurize thioureas to prepare multifunctional carbodiimides, i.e., with more than two carbodiimide groups, is known but is reported to be very difficult (Wagner, et al., Angew. Chem., 70, 819 (1981)). The art is summarized in Chem. Rev., 81, 589 (1981).
The use of polydisperse polycarbodiimides as cross-linkers for carboxylated latex resins and neutralized carboxylated water-soluble polymers is known in the art. Specifically, co-assigned U.S. application Ser. Nos. 691,378, filed Jan. 15, 1985 and 001,883, filed Jan. 9, 1987 (the disclosure of which is incorporated herein by reference), teaches the preparation of useful polycarbodiimides, useful as cross-linkers in carboxyl-containing resin systems, from certain mono-, di-, and tri-functional cycloaliphatic or saturated aliphatic isocyanates, in which the mono- and diisocyanates are optional, and co-assigned U.S. Pat. No. 4,487,964 discloses a method for the preparation of useful polycarbodiimide cross-linkers from mixed aromatic/aliphatic isocyanates. Of particular interest are the monodisperse, multifunctional carbodiimides disclosed in my co-pending, co-assigned U.S. patent application Ser. No. 747,190, filed June 21, 1985, which will be referred to in detail below.
The resin systems in which carbodiimide cross-linkers are particularly useful are those in which the material contains reactive carboxyl groups, such as are typically found in aqueous latexes used for coatings. In the past, in order to prepare the cross-linking carbodiimide in compatible emulsion form, it has been necessary to subject the carbodiimide to high-energy shear forces in the presence of relatively high concentrations of surfactants. Usually, this has required the employment of special, high-shear mixing equipment, and has exacted the further penalty of high energy costs. Since such emulsions are often difficult to keep stabilized during storage, it has often been necessary to avoid storage and shipment of the carbodiimide emulsion and, instead, to suffer the additional inconvenience of having to add emulsification equipment to the coating line.
The present invention effectively solves many of those problems of the prior art by providing polycarbodiimides which are relatively easy and inexpensive to emulsify, and in many cases, are virtually self-emulsifying. They are readily compatible with commercial, carboxyl-containing latexes, and are quite stable. In the event that it is considered convenient to utilize an emulsification step in-line, the emulsion may be effectively prepared using relatively simple mixing equipment, rather than expensive, high-shear devices.
According to the present invention there is provided a surface-active polycarbodiimide of the formula EQU R--X--R' (I)
wherein R is a residue of a hydrophobic organic compound containing at least two carbodiimide groups; R' is a residue of an organic compound having a hydrophilic segment and at least one functional group capable of reacting with a carbodiimide group; and X is a group formed by the reaction of a said carbodiimide group with a said functional group.
Likewise, there is provided a cross-linker emulsion comprising an admixture, in aqueous medium, of (a) a hydrophobic polycarbodiimide, and (b) a polycarbodiimide of formula (I).
Similarly, there is provided a cross-linkable, carboxyl-containing emulsion (e.g., a latex) or a neutralized, carboxylated, water-soluble organic resin, in combination with the cross-linker emulsion described above, as well as the cross-linked product thereof.
In addition, this invention provides methods for making the surface-active polycarbodiimide of formula I, for making the cross-linker and cross-linkable emulsions, and for cross-linking such emulsions by allowing volatilization of certain materials, to produce a cross-linked product.
As used herein, all defined groups are intended to include such groups containing any substitution which does not significantly interfere with the use of the carbodiimides for their intended purpose. Likewise, all aromatic groups are intended to include fused aromatic rings as well as substituted aromatic groups.
Also as used herein, the term "polycarbodiimide" is intended to include both monodisperse and polydisperse types of carbodiimides. The terms "multifunctional" and "polyfunctional," to the extent used, are used interchangeably and only in reference to monodisperse, polycarbodiimides.